A background art in the present technical field is Patent Literature 1 (JP-A-9-156120). This publication describes “an engaging member 31 engages with an upper opening 30 of an ink container 11. An ink replenishment port 36 of an ink replenishment container 35 includes a screw 37 on an outer circumferential portion thereof and a spring plug is press-fitted into an opening thereof. The spring plug is constructed of a body 38, a plug 41 and a spring 42, the body 38 is provided with an ink outlet 39 at the center thereof and a ring-shaped protrusion 40 is formed therearound. The spring 42 is built into the body 38 in a compressed state, urging the plug 41 toward the protrusion 40. The plug 41 includes a plurality of grooves 43 forming passages of air and ink when pushed up. The engaging member 31 includes a screw groove 32 that receives the screw 37 of the ink replenishment port 36 formed on the inner surface, four ink inlets 34 on the base plate and is provided at the center with a butting portion 33 against which the spring plug 41 butts.” (See Abstract.)